The Central Intelligence Agency website was brought down for several hours in
what appeared to be a concerted attack by hackers.

The shadowy cyber hacking group Anonymous, which has organised a series of strikes on high-profile websites, appeared to claim credit for the latest incident in online messages.

It was impossible to access the www.cia.gov home page on Friday evening after what appeared to be a denial-of-service (Dos) attack. The site was up again but running slowly on Saturday morning.

This Dos tactic usually involved bombarding a computer network with so many requests for information that its servers are overwhelmed, leaving it unable to function properly.

There is no initial suggestion that the CIA's internal system was hacked to obtain confidential information or that security was compromised.

But it is nonetheless an embarrassment for America's premier espionage agency that hackers could disable its public website. The CIA's home page was also brought down in June 2011 by Lulz Security, a group affiliated with Anonymous.

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A message posted on Friday on a Twitter page and on Tumblr feed affiliated with Anonymous celebrated the blocking of the site.

"CIA Tango Down" read the posting, using US special forces terminology for killing an enemy.

Jennifer Youngblood, a CIA spokeswoman, said on Friday night: "We are aware of the problems accessing our website, and are working to resolve them."

Anonymous claimed responsibility for shutting down the homepages of the Department of Justice and FBI last month in retaliation for the US government closing the controversial Megaupload filesharing websites.

Alabama state and Mexican mining company websites were also hacked on Friday. Web pages linked to Anonymous claimed responsibility for both attacks.

Earlier this month, a conference call between Scotland Yard and the FBI on tacking computer hacking was intercepted and published by a member of the Anonymous group.